1. Field of the Invention
The field of the invention is spatial light modulators, and devices which contain such spatial light modulators, especially holographic display devices.
2. Technical Background
Spatial light modulators (SLMs) are known from the prior art. There are various types of SLMs, based on various physical principles. SLMs are optical devices that modulate an incident light field in a spatial pattern in order to reflect or to transmit an image or to generate a holographic reconstruction corresponding to an electrical or optical input. An SLM typically comprises a one- or two-dimensional array of addressable elements (pixels) which are capable of transmitting or reflecting incident light fields. Well-established examples are liquid crystal (LC) based modulators, in which a voltage-induced birefringence is used to modulate either the amplitude or phase of an incident light field. Spatial light modulators are used in almost all areas of optical technologies and optical information processing which take advantage of variable or adaptive optical components. The applications of spatial light modulators range from display and projection systems, to microscopy, beam and wave front shaping, optical metrology, maskless lithography, ultra-fast laser pulse modulation to aberration correction in terrestrial telescopes.
Various types of SLMs are known from the prior art. These include electrically addressable SLMs (EASLMs), optically addressable SLMs (OASLMs) and magneto-optical SLMs (MOSLMs), for example.
SLMs may comprise an array of pixels. The term “pixel” derives from “picture element” and hence is a term associated with digital imaging. In the context of SLMs, a “pixel” is the hardware element which controls the display of a picture element of an image which may be seen by a viewer. The image seen by a viewer may be a holographic representation of a three-dimensional scene.
Prior art SLMs have various drawbacks. Most of the liquid-crystal-based spatial light modulators which are commercially available today exhibit refresh rates in a range of 60-120 Hz, which correspond to response times greater than 8 milliseconds. Such switching speeds are sufficient for many applications. However, there are many applications which require a much faster switching, i.e. higher frame rates. This includes in particular applications which involve time multiplexing methods. Possible applications of time multiplexing are displays that present different information to different observers. Such displays redirect the light to different observers and simultaneously change the information content of the display designated for each observer. As long as the refresh rate per observer is more than about 60 Hz, i.e. the response time is below 17 ms, the observer does not perceive any flickering of the image displayed. Examples of possible applications are automotive displays, where the driver wishes to see the navigation system whereas another passenger wishes to see a movie. Another example is 3D autostereoscopic displays where every observer wishes to see the 3D scene from their own perspective.
An object of the implementations disclosed in this document is to modulate the amplitude, or the phase, or the amplitude and phase of a light field spatially, where the temporal modulation of the desired values is fast compared with LC-based SLMs. The amplitude is typically adjustable in the entire codomain (from 0 to 1, inclusive), whereas the phase is typically adjustable in the entire codomain (from 0 to 2π, inclusive) and the target refresh rate lies within the range of between some hundred Hertz and some kHz, i.e. a response time of 5 milliseconds or less, but typically greater than or equal to 100 microseconds. A further object of the implementations is to cover the entire amplitude and/or phase range by a relative change of the amplitude and/or phase values between the individual pixels of a plane one- or two dimensional array.
It will be appreciated by those skilled in the art that the SLMs conforming to this invention may be used in any known application in which SLMs are employed. While the applications of the spatial light modulators described here are not limited to holographic displays, holographic displays are the preferred application of the spatial light modulators described here. It will be appreciated by those skilled in the art that the SLMs described herein may be used in any known form of holographic display. However, the preferred approach of the applicant to generating computer-generated video holograms will be described below.
Computer-generated video holograms (CGHs) are encoded in one or more spatial light modulators (SLMs); the SLMs may include electrically or optically controllable cells. The cells modulate the amplitude and/or phase of light by encoding hologram values corresponding to a video-hologram. The CGH may be calculated e.g. by coherent ray tracing, by simulating the interference between light reflected by the scene and a reference wave, or by Fourier or Fresnel transforms; CGH calculation methods are described for example in US2006/055994 and in US2006/139710, which are incorporated by reference. An ideal SLM would be capable of representing arbitrary complex-valued numbers, i.e. of separately controlling the amplitude and the phase of an incoming light wave. However, a typical SLM controls only one property, either amplitude or phase, with the undesirable side effect of also affecting the other property. There are different ways to spatially modulate the light in amplitude or phase, e.g. electrically addressed liquid crystal SLM, optically addressed liquid crystal SLM, magneto-optical SLM, micro mirror devices or acousto-optic modulators. The modulation of the light may be spatially continuous or composed of individually addressable cells, one-dimensionally or two-dimensionally arranged, binary, multi-level or continuous.
In the present document, the term “encoding” denotes the way in which regions of a spatial light modulator are supplied with control values to encode a hologram so that a 3D-scene can be reconstructed from the SLM.
In contrast to purely auto-stereoscopic displays, with video holograms an observer sees an optical reconstruction of a light wave front of a three-dimensional scene. The 3D-scene is reconstructed in a space that stretches between the eyes of an observer and the spatial light modulator (SLM), or possibly even behind the SLM. The SLM can also be encoded with video holograms such that the observer sees objects of a reconstructed three-dimensional scene in front of the SLM and other objects on or behind the SLM.
The cells of the spatial light modulator may be transmissive cells which are passed through by light, the rays of which are capable of generating interference at least at a defined position and over a spatial coherence length of a few millimetres. This allows holographic reconstruction with an adequate resolution in at least one dimension. This kind of light will be referred to as ‘sufficiently coherent light’. However, cells which operate in a reflective geometry are also possible.
In order to ensure sufficient temporal coherence, the spectrum of the light emitted by the light source must be limited to an adequately narrow wavelength range, i.e. it must be near-monochromatic. The spectral bandwidth of high-brightness LEDs is sufficiently narrow to ensure temporal coherence for holographic reconstruction. The diffraction angle at the SLM is proportional to the wavelength, which means that only a monochromatic source will lead to a sharp reconstruction of object points. A broadened spectrum will lead to broadened object points and smeared object reconstructions. The spectrum of a laser source can be regarded as monochromatic. The spectral line width of a LED is sufficiently narrow to facilitate good reconstructions.
Spatial coherence relates to the lateral extent of the light source. Conventional light sources, like LEDs or Cold Cathode Fluorescent Lamps (CCFLs), can also meet these requirements if they radiate light through an adequately narrow aperture. Light from a laser source can be regarded as emanating from a point source within diffraction limits and, depending on the modal purity, leads to a sharp reconstruction of the object, i.e. each object point is reconstructed as a point within diffraction limits.
Light from a spatially incoherent source is laterally extended and causes a smearing of the reconstructed object. The amount of smearing is given by the broadened size of an object point reconstructed at a given position. In order to use a spatially incoherent source for hologram reconstruction, a trade-off has to be found between brightness and limiting the lateral extent of the source with an aperture. The smaller the light source, the better is its spatial coherence.
A line light source can be considered to be a point light source if seen from a right angle to its longitudinal extension. Light waves can thus propagate coherently in that direction, but incoherently in all other directions.
In general, a hologram reconstructs a scene holographically by coherent superposition of waves in the horizontal and the vertical directions. Such a video hologram is called a full-parallax hologram. The reconstructed object can be viewed with motion parallax in the horizontal and the vertical directions, like a real object. However, a large viewing angle requires high resolution in both the horizontal and the vertical direction of the SLM.
Often, the requirements on the SLM are lessened by restriction to a horizontal-parallax-only (HPO) hologram. The holographic reconstruction takes place only in the horizontal direction, whereas there is no holographic reconstruction in the vertical direction. This results in a reconstructed object with horizontal motion parallax. The perspective view does not change upon vertical motion. A HPO hologram requires less resolution of the SLM in the vertical direction than a full-parallax hologram. A vertical-parallax-only (VPO) hologram is also possible but uncommon. The holographic reconstruction occurs only in the vertical direction and results in a reconstructed object with vertical motion parallax. There is no motion parallax in the horizontal direction. The different perspective views for the left eye and right eye have to be created separately.
In some of the implementations described herein, electrowetting cells are used. An early use of the term “electrowetting” was in 1981; “electrowetting” was used in G. Beni and S. Hackwood, Appl. Phys. Lett. 38, 4, pp. 207-209 (1981). The electrowetting effect was originally defined as “the change in solid electrolyte contact angle due to an applied potential difference between the solid and the electrolyte”. Since then a number of devices based on electrowetting have been devised. The phenomenon of electrowetting can be understood in terms of the forces that result from the applied electric field. The fringing field at the corners of the electrolyte droplet tend to pull the droplet down onto the electrode, lowering the macroscopic contact angle, and increasing the droplet contact area. Alternatively electrowetting can be viewed from a thermodynamic perspective. Since the surface tension of an interface is defined as the Gibbs free energy required to create a certain area of that surface, it contains both chemical and electrical components. The chemical component is just the natural surface tension of the solid/electrolyte interface with no electric field. The electrical component is the energy stored in the capacitor formed between the conductor and the electrolyte. In the present document the term ‘electrowetting cell’ describes in particular a single optical element changing the amplitude and/or phase of a wave field. The electrowetting cell includes a chamber having cell walls filled with at least two different non-miscible fluids or liquids, especially a conductive polar fluid or liquid, like water, and an insulating non-conductive fluid or liquid, like oil. It is noted and understood that a fluid can be a liquid or a gas. In general, a fluid is a subset of the phases of matter and include liquid, (saturated) gas, plasma and, to some extent, plastic solid. It is noted that the term “electrowetting” within the context of this document is also to be understood as “electrowetting-on-dielectrics” (EWOD).
3. Discussion of Related Art
WO 2004/044659 (US2006/0055994) filed by the applicant describes a device for reconstructing three-dimensional scenes by way of diffraction of sufficiently coherent light; the device includes a point light source or line light source, a lens for focusing the light and a spatial light modulator. In contrast to conventional holographic displays, the SLM in transmission mode reconstructs a 3D-scene in at least one ‘virtual observer window’ (see Appendix I and II for a discussion of this term and the related technology). Each virtual observer window is situated near the observer's eyes and is restricted in size so that the virtual observer windows are situated in a single diffraction order, so that each eye sees the complete reconstruction of the three-dimensional scene in a frustum-shaped reconstruction space, which stretches between the SLM surface and the virtual observer window. To allow a holographic reconstruction free of disturbance, the virtual observer window size must not exceed the periodicity interval of one diffraction order of the reconstruction. However, it must be at least large enough to enable a viewer to see the entire reconstruction of the 3D-scene through the window(s). The other eye can see through the same virtual observer window, or is assigned a second virtual observer window, which is accordingly created by a second light source. Here, a visibility region i.e. the range of positions from which an observer can see a correct reconstruction, which would be rather large, is limited to the locally positioned virtual observer windows. This virtual observer window solution uses the larger area and high resolution of a conventional SLM surface to generate a reconstruction which is viewed from a smaller area which is the size of the virtual observer windows. This leads to the effect that the diffraction angles, which are small due to geometrical reasons, and the resolution of current generation SLMs, are sufficient to achieve a high-quality real-time holographic reconstruction using reasonable, consumer level computing equipment.
A mobile phone which generates a three dimensional image is disclosed in US2004/0223049. However, the three dimensional image disclosed in US2004/0223049 is generated using autostereoscopy. One problem with autostereoscopically generated three dimensional images is that typically the viewer perceives the image to be inside the display, whereas the viewer's eyes tend to focus on the surface of the display. This disparity between where the viewer's eyes focus and the perceived position of the three dimensional image leads to viewer discomfort after some time in many cases. This problem does not occur, or is significantly reduced, in the case of three dimensional images generated by holography.